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<html><body><div class="content"><div class="section" id="the-try-statement">
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<span id="finally"></span><span id="except"></span><span id="try"></span><h2>The <a class="reference internal" href="#try"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></a> statement</h2>
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<span class="target" id="index-10"></span><p id="index-11">The <a class="reference internal" href="#try"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></a> statement specifies exception handlers and/or cleanup code
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for a group of statements:</p>
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<pre>
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<strong id="grammar-token-try_stmt">try_stmt </strong> ::= try1_stmt | try2_stmt
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<strong id="grammar-token-try1_stmt">try1_stmt</strong> ::= "try" ":" <a class="reference internal" href="#grammar-token-suite"><tt class="xref docutils literal"><span class="pre">suite</span></tt></a>
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("except" [<a class="reference internal" href="expressions.html#grammar-token-expression"><tt class="xref docutils literal"><span class="pre">expression</span></tt></a> ["as" <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><tt class="xref docutils literal"><span class="pre">identifier</span></tt></a>]] ":" <a class="reference internal" href="#grammar-token-suite"><tt class="xref docutils literal"><span class="pre">suite</span></tt></a>)+
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["else" ":" <a class="reference internal" href="#grammar-token-suite"><tt class="xref docutils literal"><span class="pre">suite</span></tt></a>]
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["finally" ":" <a class="reference internal" href="#grammar-token-suite"><tt class="xref docutils literal"><span class="pre">suite</span></tt></a>]
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<strong id="grammar-token-try2_stmt">try2_stmt</strong> ::= "try" ":" <a class="reference internal" href="#grammar-token-suite"><tt class="xref docutils literal"><span class="pre">suite</span></tt></a>
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"finally" ":" <a class="reference internal" href="#grammar-token-suite"><tt class="xref docutils literal"><span class="pre">suite</span></tt></a>
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</pre>
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<p>The <a class="reference internal" href="#except"><tt class="xref std std-keyword docutils literal"><span class="pre">except</span></tt></a> clause(s) specify one or more exception handlers. When no
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exception occurs in the <a class="reference internal" href="#try"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></a> clause, no exception handler is executed.
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When an exception occurs in the <a class="reference internal" href="#try"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></a> suite, a search for an exception
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handler is started. This search inspects the except clauses in turn until one
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is found that matches the exception. An expression-less except clause, if
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present, must be last; it matches any exception. For an except clause with an
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expression, that expression is evaluated, and the clause matches the exception
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if the resulting object is “compatible” with the exception. An object is
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compatible with an exception if it is the class or a base class of the exception
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object or a tuple containing an item compatible with the exception.</p>
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<p>If no except clause matches the exception, the search for an exception handler
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continues in the surrounding code and on the invocation stack. <a class="footnote-reference" href="#id5" id="id1">[1]</a></p>
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<p>If the evaluation of an expression in the header of an except clause raises an
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exception, the original search for a handler is canceled and a search starts for
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the new exception in the surrounding code and on the call stack (it is treated
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as if the entire <a class="reference internal" href="#try"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></a> statement raised the exception).</p>
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<p>When a matching except clause is found, the exception is assigned to the target
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specified after the <a class="reference internal" href="#as"><tt class="xref std std-keyword docutils literal"><span class="pre">as</span></tt></a> keyword in that except clause, if present, and
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the except clause’s suite is executed. All except clauses must have an
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executable block. When the end of this block is reached, execution continues
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normally after the entire try statement. (This means that if two nested
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handlers exist for the same exception, and the exception occurs in the try
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clause of the inner handler, the outer handler will not handle the exception.)</p>
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<p>When an exception has been assigned using <tt class="docutils literal"><span class="pre">as</span> <span class="pre">target</span></tt>, it is cleared at the
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end of the except clause. This is as if</p>
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<div class="highlight-python3"><div class="highlight"><pre><span class="k">except</span> <span class="n">E</span> <span class="k">as</span> <span class="n">N</span><span class="p">:</span>
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<span class="n">foo</span>
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</pre></div>
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</div>
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<p>was translated to</p>
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<div class="highlight-python3"><div class="highlight"><pre><span class="k">except</span> <span class="n">E</span> <span class="k">as</span> <span class="n">N</span><span class="p">:</span>
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<span class="k">try</span><span class="p">:</span>
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<span class="n">foo</span>
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<span class="k">finally</span><span class="p">:</span>
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<span class="k">del</span> <span class="n">N</span>
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</pre></div>
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</div>
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<p>This means the exception must be assigned to a different name to be able to
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refer to it after the except clause. Exceptions are cleared because with the
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traceback attached to them, they form a reference cycle with the stack frame,
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keeping all locals in that frame alive until the next garbage collection occurs.</p>
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<p id="index-12">Before an except clause’s suite is executed, details about the exception are
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stored in the <a class="reference internal" href="../library/sys.html#module-sys" title="sys: Access system-specific parameters and functions."><tt class="xref py py-mod docutils literal"><span class="pre">sys</span></tt></a> module and can be accessed via <a class="reference internal" href="../library/sys.html#sys.exc_info" title="sys.exc_info"><tt class="xref py py-func docutils literal"><span class="pre">sys.exc_info()</span></tt></a>.
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<a class="reference internal" href="../library/sys.html#sys.exc_info" title="sys.exc_info"><tt class="xref py py-func docutils literal"><span class="pre">sys.exc_info()</span></tt></a> returns a 3-tuple consisting of the exception class, the
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exception instance and a traceback object (see section <a class="reference internal" href="datamodel.html#types"><em>The standard type hierarchy</em></a>) identifying
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the point in the program where the exception occurred. <a class="reference internal" href="../library/sys.html#sys.exc_info" title="sys.exc_info"><tt class="xref py py-func docutils literal"><span class="pre">sys.exc_info()</span></tt></a>
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values are restored to their previous values (before the call) when returning
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from a function that handled an exception.</p>
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<p id="index-13">The optional <a class="reference internal" href="#else"><tt class="xref std std-keyword docutils literal"><span class="pre">else</span></tt></a> clause is executed if and when control flows off
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the end of the <a class="reference internal" href="#try"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></a> clause. <a class="footnote-reference" href="#id6" id="id2">[2]</a> Exceptions in the <a class="reference internal" href="#else"><tt class="xref std std-keyword docutils literal"><span class="pre">else</span></tt></a>
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clause are not handled by the preceding <a class="reference internal" href="#except"><tt class="xref std std-keyword docutils literal"><span class="pre">except</span></tt></a> clauses.</p>
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<p id="index-14">If <a class="reference internal" href="#finally"><tt class="xref std std-keyword docutils literal"><span class="pre">finally</span></tt></a> is present, it specifies a ‘cleanup’ handler. The
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<a class="reference internal" href="#try"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></a> clause is executed, including any <a class="reference internal" href="#except"><tt class="xref std std-keyword docutils literal"><span class="pre">except</span></tt></a> and
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<a class="reference internal" href="#else"><tt class="xref std std-keyword docutils literal"><span class="pre">else</span></tt></a> clauses. If an exception occurs in any of the clauses and is
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not handled, the exception is temporarily saved. The <a class="reference internal" href="#finally"><tt class="xref std std-keyword docutils literal"><span class="pre">finally</span></tt></a> clause
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is executed. If there is a saved exception it is re-raised at the end of the
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<a class="reference internal" href="#finally"><tt class="xref std std-keyword docutils literal"><span class="pre">finally</span></tt></a> clause. If the <a class="reference internal" href="#finally"><tt class="xref std std-keyword docutils literal"><span class="pre">finally</span></tt></a> clause raises another
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exception, the saved exception is set as the context of the new exception.
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If the <a class="reference internal" href="#finally"><tt class="xref std std-keyword docutils literal"><span class="pre">finally</span></tt></a> clause executes a <a class="reference internal" href="simple_stmts.html#return"><tt class="xref std std-keyword docutils literal"><span class="pre">return</span></tt></a> or <a class="reference internal" href="simple_stmts.html#break"><tt class="xref std std-keyword docutils literal"><span class="pre">break</span></tt></a>
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statement, the saved exception is discarded:</p>
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<div class="highlight-python3"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">def</span> <span class="nf">f</span><span class="p">():</span>
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<span class="gp">... </span> <span class="k">try</span><span class="p">:</span>
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<span class="gp">... </span> <span class="mi">1</span><span class="o">/</span><span class="mi">0</span>
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<span class="gp">... </span> <span class="k">finally</span><span class="p">:</span>
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<span class="gp">... </span> <span class="k">return</span> <span class="mi">42</span>
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<span class="gp">...</span>
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<span class="gp">>>> </span><span class="n">f</span><span class="p">()</span>
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<span class="go">42</span>
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</pre></div>
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</div>
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<p>The exception information is not available to the program during execution of
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the <a class="reference internal" href="#finally"><tt class="xref std std-keyword docutils literal"><span class="pre">finally</span></tt></a> clause.</p>
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<p id="index-15">When a <a class="reference internal" href="simple_stmts.html#return"><tt class="xref std std-keyword docutils literal"><span class="pre">return</span></tt></a>, <a class="reference internal" href="simple_stmts.html#break"><tt class="xref std std-keyword docutils literal"><span class="pre">break</span></tt></a> or <a class="reference internal" href="simple_stmts.html#continue"><tt class="xref std std-keyword docutils literal"><span class="pre">continue</span></tt></a> statement is
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executed in the <a class="reference internal" href="#try"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></a> suite of a <a class="reference internal" href="#try"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></a>...<a class="reference internal" href="#finally"><tt class="xref std std-keyword docutils literal"><span class="pre">finally</span></tt></a>
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statement, the <a class="reference internal" href="#finally"><tt class="xref std std-keyword docutils literal"><span class="pre">finally</span></tt></a> clause is also executed ‘on the way out.’ A
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<a class="reference internal" href="simple_stmts.html#continue"><tt class="xref std std-keyword docutils literal"><span class="pre">continue</span></tt></a> statement is illegal in the <a class="reference internal" href="#finally"><tt class="xref std std-keyword docutils literal"><span class="pre">finally</span></tt></a> clause. (The
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reason is a problem with the current implementation — this restriction may be
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lifted in the future).</p>
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<p>The return value of a function is determined by the last <a class="reference internal" href="simple_stmts.html#return"><tt class="xref std std-keyword docutils literal"><span class="pre">return</span></tt></a>
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statement executed. Since the <a class="reference internal" href="#finally"><tt class="xref std std-keyword docutils literal"><span class="pre">finally</span></tt></a> clause always executes, a
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<a class="reference internal" href="simple_stmts.html#return"><tt class="xref std std-keyword docutils literal"><span class="pre">return</span></tt></a> statement executed in the <a class="reference internal" href="#finally"><tt class="xref std std-keyword docutils literal"><span class="pre">finally</span></tt></a> clause will
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always be the last one executed:</p>
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<div class="highlight-python3"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">def</span> <span class="nf">foo</span><span class="p">():</span>
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<span class="gp">... </span> <span class="k">try</span><span class="p">:</span>
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<span class="gp">... </span> <span class="k">return</span> <span class="s">'try'</span>
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<span class="gp">... </span> <span class="k">finally</span><span class="p">:</span>
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<span class="gp">... </span> <span class="k">return</span> <span class="s">'finally'</span>
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<span class="gp">...</span>
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<span class="gp">>>> </span><span class="n">foo</span><span class="p">()</span>
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<span class="go">'finally'</span>
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</pre></div>
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</div>
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<p>Additional information on exceptions can be found in section <a class="reference internal" href="executionmodel.html#exceptions"><em>Exceptions</em></a>,
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and information on using the <a class="reference internal" href="simple_stmts.html#raise"><tt class="xref std std-keyword docutils literal"><span class="pre">raise</span></tt></a> statement to generate exceptions
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may be found in section <a class="reference internal" href="simple_stmts.html#raise"><em>The raise statement</em></a>.</p>
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</div>
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</div></body></html> |